wiping out cattails and other marginal plants is tough. you need to be very consistant with your treatments. dont expect to spray one time and eradicate the problem. try aquatic label glyphosate (same active ingredient thats in roundup) or a less expensive solution is any aquatic label 2,4-D. glyphosate tends to work better for me, I treat the weeds monthly and usually dont eradicate the problem but I do get good control. If you find out exactly what type of "willow type bushes" you are dealing with I can be a bit more specific about the treatment. both the above mentioned herbicides are broad range and kill just about anything you spray them on, so be careful of your drift if you have any plants you would like to keep in the area.
hope this helps
